Despite the recognized benefits of regular physical activity, older adults remain among the most inactive segments of the U.S. population. Over the past two decades, effective telephone-based counseling approaches to promote and sustain physical activity among older adults have been developed and validated. The next step in this line of research is to systematically evaluate methods for extending the reach and applicability of the telephone model for physical activity promotion through use of trained peer volunteers. No systematic evaluation of peer- vs. health educator-based delivery of physical activity counseling currently exists. The objective of this study is to conduct a randomized controlled trial to evaluate the effects of a 12-month telephone-supervised, home-based physical activity intervention, delivered via health educators versus supervised, trained peers, on physical activity patterns. Inactive men and women ages 50 years and older (N = 180) will be randomly assigned to one of these two programs or to an attention-control arm. The three arms will be matched with respect to the amount of counselor contact. Data will be collected at baseline, 6, and 12 months using age-appropriate physical activity, physical performance, and quality of life measures. Our primary hypotheses are that persons assigned to either experimental arm will show greater 12-month improvement in physical activity levels relative to controls, and both arms will show 12-month physical activity levels that are significantly above baseline levels. Additional questions of interest involve evaluation of potential mediators and moderators (including environmental influences) as well as the costs of the two physical activity interventions, and intervention-related effects on physical functioning and health-related quality of life. This study will contribute important public health information related to methods for expanding the scope and potential impact of the telephone model to a larger segment of the older adult population.